This application is related to U.S. patent application entitled xe2x80x9cLaser Writing Method and Apparatusxe2x80x9d, identified as Ser. No. 09/322,803, filed concurrent herewith; and U.S. Patent Application entitled xe2x80x9cQuasi-Continuous Wave Lithography Apparatus and Methodxe2x80x9d, identified as Ser. No. 09/322,121, filed concurrent herewith, which applications are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method and apparatus to detect a flaw in a surface of an article, and more particularly to a surface inspection apparatus that uses a diode-pumped, mode-locked laser.
2. Description of Related Art
In the process of manufacturing a silicon microchip, light is directed through a reticle mask to etch circuits into a silicon wafer disc. The presence of dirt, dust, smudges, scratches or other flaws on the surface of the silicon wafer is highly undesirable and will adversely affect the resulting circuits. As a result, the silicon wafers are necessarily inspected prior to and during the manufacturing process. One common inspection technique is for a human inspector to visually examine the surface under intense light and magnification. However, the microscope has a small field of view so it takes a human inspector an extended period of time to visually examine the entire surface of the wafer.
Laser surface inspection devices have been developed for inspecting the surface of polished silicon wafers to accurately detect small particles or flaws. Examples of such devices are disclosed in Alford et al. U.S. Pat. No. 4,376,583 issued Mar. 15, 1983 and Moran U.S. Pat. No. 4,630,276 issued Dec. 16, 1986. In these known laser surface inspection systems, a laser beam is transversed across the surface of the silicon wafer and the reflections from the wafer are collected and analyzed to provide information about any flaws present on the wafer surface. The light is specularly reflected from the polished surface of the wafer, but in locations where the beam strikes surface flaws, the light is scattered. By separately collecting the scattered and specularly reflected light, the inspection device can quickly determine the size and locations of flaws on the surface of the wafer. This provides a satisfactory pass/fail test for inspecting the wafers, however, the nature and source of the flaws are not suitably analyzed by such laser inspection techniques. Also, when the wafer has been etched with a pattern to form the microchips the etching may provide spurious indications of flaws on the surface.
To inspect the patterned surface of silicon wafers, low angle laser surface inspection devices are employed, such as those disclosed in Koizumi et al. U.S. Pat. No. 4,614,427 and Shiba et al. U.S. Pat. No. 4,669,875 for example. These devices inspect the surfaces of patterned wafers using a laser beam at a low glancing angle. However, laser scanning does not provide sufficient resolution or clarity of the flaws to analyze the nature or source of the flaws in the surface.
Optical scanning arrangements are known which use optical lenses to microscopically view the surface and identify and analyze flaws in the surface. However, such systems produce enormous amounts of data and require powerful computers to process and analyze the data produced. Accordingly such systems are very expensive. Because of the small field of view and the enormous volume of data obtained, this type of system is relatively low.
In one recently developed device, Hitachi model HLIS-200, foreign particles are detected by a low angle fixed spot laser beam. As the wafer rotates and translates under the laser beam, the particles are detected by an overhead photomultiplier and a map of the particles is formed. Subsequently, the foreign particles may be microscopically observed and photographed by repositioning the flaw under a microscopic viewing device. However, this requires a very accurate and reliable X-Y table to reposition the flaws in the field of view. Also, the process of inspecting the wafers by moving the wafer around under the fixed spot laser is slow and time consuming.
Accordingly, an object of the invention is to provide a method and apparatus to inspect a surface of an article.
Another object of the invention is to provide a method and apparatus to detect flaws in a surface of an article.
Yet another object of the invention is to provide a method and apparatus to detect flaws in a surface of an integrated circuit.
Still another object of the invention is to provide a method and apparatus to detect flaws in a surface of an article using a diode-pumped, mode-locked laser.
These and other objects of the invention are achieved in an inspection apparatus that includes a laser system. The laser system has a high reflector and an output coupler that define an oscillator cavity which produces an output beam. A gain medium and a mode locking device are positioned in the oscillator cavity. A diode pump source produces a pump beam that is incident on the gain medium. An output beam directing apparatus directs the output beam to the surface of the article. A surface flaw at the surface of the article produces scattered light from at least a portion of the output beam incident on the surface flaw. A detector is positioned to detect the scattered light.
In another embodiment, the laser system includes a first amplifier.
In another embodiment, a method is provided for detecting flaws in a surface of an article. A diode-pumped laser system is provided that includes an oscillator cavity, a gain medium and a mode locking device positioned in the oscillator cavity. An output beam is produced from the laser system. The output beam is directed to the surface of the article. Light scattered from a flaw on the surface of the article is detected.